docs(firestore): fix typo (#4142)

Co-authored-by: Mike Diarmid <[email protected]>

docs/firestore/usage.mdx

@@ -542,7 +542,7 @@ should be used.
 When using transactions, note that:
 
 - Read operations must come before write operations
-- Transactions will fail is the client is offline, they cannot use cached data
+- Transactions will fail when the client is offline, they cannot use cached data
 
 An example of where a transaction could be used would be in an application where a user can subscribe to a channel. When
 a user presses the subscribe button, a "subscribers" field in a document increments. Without using Transactions, we would