blog: fix spelling (an vs and) (#3203)

Author: sam-github

locale/en/blog/vulnerability/june-2020-security-releases.md

@@ -49,7 +49,7 @@ exists in the UnicodeString::doAppend() function in common/unistr.cpp.
 Fix was applied to 10.x in an abundance of caution, even though there is no
 known way to trigger the overflow in 10.x.
 
-Does not affect 12.x or 14.x, they do not include and affected version of ICU.
+Does not affect 12.x or 14.x, they do not include an affected version of ICU.
 
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