Added grayscale to custom words

Signed-off-by: ClemensLinnhoff <clemens.linnhoff@partner.bmw.de>

Author: ClemensLinnhoff

.github/spelling_custom_words_en_US.txt

@@ -146,6 +146,7 @@ gesetze
 github
 glichkeit
 GNSS
+grayscale
 gridded
 Groessen
 GroundTruthInit