Apply two minor fixes to rgbasm(5)

Mustn't → must not
Add a comma to INCBIN sentence to mirror INCLUDE's

src/asm/rgbasm.5

@@ -283,7 +283,7 @@ Most of them return a string, however some of these functions actually return an
 .It Sy Name Ta Sy Operation
 .It Fn STRLEN string Ta Returns the number of characters in Ar string .
 .It Fn STRCAT str1 str2 Ta Appends Ar str2 No to Ar str1 .
-.It Fn STRCMP str1 str2 Ta Returns negative if Ar str1 No is alphabetically lower than Ar str2 No , zero if they match, positive if Ar str1 No is greater than Ar str2 .
+.It Fn STRCMP str1 str2 Ta Returns -1 if Ar str1 No is alphabetically lower than Ar str2 No , zero if they match, 1 if Ar str1 No is greater than Ar str2 .
 .It Fn STRIN str1 str2 Ta Returns the position of Ar str2 No in Ar str1 No or zero if it's not present Pq first character is position 1 .
 .It Fn STRSUB str pos len Ta Returns a substring from Ar str No starting at Ar pos Po first character is position 1 Pc and Ar len No characters long.
 .It Fn STRUPR str Ta Converts all characters in Ar str No to capitals and returns the new string.
@@ -743,7 +743,7 @@ Symbol names can contain letters, numbers, underscores, hashes and
 However, they must begin with either a letter, a number, or an underscore.
 Periods are allowed exclusively for labels, as described below.
 A symbol cannot have the same name as a reserved keyword.
-.Em \&In the line where a symbol is defined there mustn't be any whitespace before it ,
+.Em \&In the line where a symbol is defined there must not be any whitespace before it ,
 otherwise
 .Nm
 will treat it as a macro invocation.
@@ -1425,7 +1425,7 @@ immediately aborts.
 Use
 .Ic INCLUDE
 to process another assembler file and then return to the current file when done.
-If the file isn't found in the current directory the include path list (see the
+If the file isn't found in the current directory, the include path list (see the
 .Fl i
 option in
 .Xr rgbasm 1 )