[analyzer] Fix memory error bug category capitalization.

[analyzer] Fix memory error bug category capitalization.

It was written as "Memory Error" in most places and as "Memory error" in a few
other places, however it is the latter that is more consistent with
other categories (such as "Logic error").

rdar://problem/31718115

Differential Revision: https://reviews.llvm.org/D32702

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302016 91177308-0d34-0410-b5e6-96231b3b80d8