chore: fix spelling

Signed-off-by: Zack Koppert <zkoppert@github.com>

Author: zkoppert

.github/linters/.markdown-lint.yml

@@ -20,7 +20,7 @@
 ###############
 # line length
 MD013: false
-# singe h1
+# single h1
 MD025: false
 # duplicate headers
 MD024: false