Fix typo in LangRef.md regarding regions (#167242)

Author: moar55

mlir/docs/LangRef.md

@@ -440,7 +440,7 @@ region is not imposed by the IR. Instead, the containing operation defines the
 semantics of the regions it contains. MLIR currently defines two kinds of
 regions: [SSACFG regions](#control-flow-and-ssacfg-regions), which describe
 control flow between blocks, and [Graph regions](#graph-regions), which do not
-require control flow between block. The kinds of regions within an operation are
+require control flow between blocks. The kinds of regions within an operation are
 described using the [RegionKindInterface](Interfaces.md/#regionkindinterfaces).
 
 Regions do not have a name or an address, only the blocks contained in a region