lib: zstd: Fix comment typo

The double `when' is duplicated in line 999, remove one.
Signed-off-by: Xin Gao <gaoxin@cdjrlc.com>
Signed-off-by: Nick Terrell <terrelln@fb.com>

lib/zstd/decompress/zstd_decompress.c

@@ -996,7 +996,7 @@ size_t ZSTD_decompress(void* dst, size_t dstCapacity, const void* src, size_t sr
 size_t ZSTD_nextSrcSizeToDecompress(ZSTD_DCtx* dctx) { return dctx->expected; }
 
 /*
- * Similar to ZSTD_nextSrcSizeToDecompress(), but when when a block input can be streamed,
+ * Similar to ZSTD_nextSrcSizeToDecompress(), but when a block input can be streamed,
  * we allow taking a partial block as the input. Currently only raw uncompressed blocks can
  * be streamed.
  *