Release connection when unprepared statements can't be built

Add specs
Fix typo in docs

src/db/pool_prepared_statement.cr

@@ -29,7 +29,7 @@ module DB
     end
 
     # builds a statement over a real connection
-    # the conneciton is registered in `@connections`
+    # the connection is registered in `@connections`
     private def build_statement
       clean_connections
       conn, existing = @db.checkout_some(@connections)

src/db/pool_unprepared_statement.cr

@@ -15,7 +15,13 @@ module DB
 
     # builds a statement over a real connection
     private def build_statement
-      @db.pool.checkout.unprepared.build(@query)
+      conn = @db.pool.checkout
+      begin
+        conn.unprepared.build(@query)
+      rescue ex
+        conn.release
+        raise ex
+      end
     end
   end
 end