doc/contribute.html: capitalize the first letter

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/109080048

doc/contribute.html

@@ -473,7 +473,7 @@ Mercurial failed to merge your changes with the independent change.
 When this happens, Mercurial leaves both edits in the file,
 marked by <code>&lt;&lt;&lt;&lt;&lt;&lt;&lt;</code> and
 <code>&gt;&gt;&gt;&gt;&gt;&gt;&gt;</code>.
-it is now your job to edit the file to combine them.
+It is now your job to edit the file to combine them.
 Continuing the example, searching for those strings in <code>flag_test.go</code>
 might turn up:
 </p>