fixed typo for build instructions

mkdir build should work when copied and pasted.

apps/popcorn/converter/README.md

@@ -6,7 +6,7 @@ To build the converter, either do a `PICO_PLATFORM=host` build of pico-playgroun
 or from this directory just do
 
 ```bash
-mkidr build
+mkdir build
 cd build
 cmake -DCMAKE_BUILD_TYPE=Release ..
 make
@@ -44,4 +44,4 @@ You can then do:
 converter movie.rgb movie.pcm movie.pl2
 ```
 
-If the inputs are not as specified, then the converter will likely crash!
+If the inputs are not as specified, then the converter will likely crash!