doc: Small tweak for consistency

submitted by "Lyle"

Author: petere

doc/src/sgml/datatype.sgml

@@ -351,7 +351,7 @@
         <entry><type>bigint</></entry>
         <entry>8 bytes</entry>
         <entry>large-range integer</entry>
-        <entry>-9223372036854775808 to 9223372036854775807</entry>
+        <entry>-9223372036854775808 to +9223372036854775807</entry>
        </row>
 
        <row>