Use 'b/break' set breakpoints rather than 'breakpoint'.

Fix issue -> Undefined command: "breakpoint".

15-video-ports/README.md

@@ -19,7 +19,7 @@ When this port is queried, it saves the result in port `0x3d5`
 
 Don't miss the opportunity to use `gdb` to inspect the value of C variables,
 since we still can't print them on the screen. To do so, set a breakpoint
-for a specific line, `breakpoint kernel.c:21` and use the `print` command
+for a specific line, `b kernel.c:21` and use the `print` command
 to examine variables. Aren't you glad now that we invested some time in
 compiling the cross-compiled gdb? ;)
 

15-video-ports/kernel/kernel.c

@@ -18,7 +18,7 @@ void main() {
     /* Now you can examine both variables using gdb, since we still
      * don't know how to print strings on screen. Run 'make debug' and
      * on the gdb console:
-     * breakpoint kernel.c:21
+     * b kernel.c:21
      * continue
      * print position
      * print offset_from_vga