doc/contribute.html: capitalize the first letter

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/109080048
diff --git a/doc/contribute.html b/doc/contribute.html
index 32ff09b..849c1e4 100644
--- a/doc/contribute.html
+++ b/doc/contribute.html
@@ -473,7 +473,7 @@
 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>