Update src/SlabSL.H

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: The9Cat

src/SlabSL.H

@@ -20,7 +20,7 @@
 /** @class SlabSL
     @brief This routine computes the potential, acceleration and density
     using expansion periodic in X & Y and outgoing vacuum boundary
-    condtions in Z
+    conditions in Z
 
     @details **YAML configuration**