Change masterthesis to mastersthesis

The question which one is "correct" is apparently not completely trivial
to decide. Apparently Mendeley uses the former (at least this is implied
on <https://tex.stackexchange.com/q/415204>, but that reference also points
out this is wrong)

The wrong name is also given at <https://www.bibtex.com/e/entry-types/>
but note that the example they give here uses the right form, as do the
templates at <https://www.bibtex.com/t/template-mastersthesis/> (I've
reported this to the company running that website)

Also the biblatex manual lists only `mastersthesis`.

But ultimately, the source of truth should be bibtex itself; so see e.g.
<http://tug.org/svn/texlive/trunk/Build/extra/bibtex/alpha.bst?view=markup#l736>

src/bibtex.jl

@@ -10,7 +10,7 @@ const Required = Union{String,Tuple{String,String}}
         "incollection"  => ["author", "booktitle", "publisher", "title", "year"]
         "inproceedings" => ["author", "booktitle", "title", "year"]
         "manual"        => ["title"]
-        "masterthesis"  => ["author", "school", "title", "year"]
+        "mastersthesis" => ["author", "school", "title", "year"]
         "misc"          => []
         "phdthesis"     => ["author", "school", "title", "year"]
         "proceedings"   => ["title", "year"]
@@ -28,7 +28,7 @@ const rules = Dict{String,Vector{Required}}([
     "incollection"  => ["author", "booktitle", "publisher", "title", "year"]
     "inproceedings" => ["author", "booktitle", "title", "year"]
     "manual"        => ["title"]
-    "mastersthesis"  => ["author", "school", "title", "year"]
+    "mastersthesis" => ["author", "school", "title", "year"]
     "misc"          => []
     "phdthesis"     => ["author", "school", "title", "year"]
     "proceedings"   => ["title", "year"]

src/constant.jl

@@ -12,7 +12,7 @@ const Fields = Dict{String,String}
         :incollection,
         :inproceedings,
         :manual,
-        :masterthesis,
+        :mastersthesis,
         :misc,
         :phdthesis,
         :proceedings,
@@ -29,7 +29,7 @@ const entries = [
     :incollection,
     :inproceedings,
     :manual,
-    :masterthesis,
+    :mastersthesis,
     :misc,
     :phdthesis,
     :proceedings,