user-manual: Fix 'both: so' -> 'both; so' typo

The clause "so `git log ...` will return no commits..." is
independent, not a description of "both", so a semicolon is more
appropriate.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -782,7 +782,7 @@ e05db0fd4f31dde7005f075a84f96b360d05984b
 
 Or you could recall that the ... operator selects all commits
 contained reachable from either one reference or the other but not
-both: so
+both; so
 
 -------------------------------------------------
 $ git log origin...master