trivial: remove leading sentence in limits

Author: kay-kim

source/reference/limits.txt

@@ -314,8 +314,7 @@ Naming Restrictions
 
 .. limit:: Database Name Case Sensitivity
 
-   Database names are case sensitive even if the underlying file
-   system is case insensitive. MongoDB does not permit database names
+   MongoDB does not permit database names
    that differ only by the case of the characters.
 
 .. limit:: Restrictions on Database Names for Windows