chore(docs): Fix a typo (us -> is)

https://datafusion.apache.org/user-guide/explain-usage.html#query-plans-are-trees

skip ci

Author: martin-g

docs/source/user-guide/explain-usage.md

@@ -131,7 +131,7 @@ physical plan in Figure 1 in tree format will look like
 Each node in the tree/plan ends with `Exec` and is sometimes also called an `operator` or `ExecutionPlan` where data is
 processed, transformed and sent up.
 
-1. First, data in parquet the `hits.parquet` file us read in parallel using 16 cores in 16 "partitions" (more on this later) from `DataSourceExec`, which applies a first pass at filtering during the scan.
+1. First, data in parquet the `hits.parquet` file is read in parallel using 16 cores in 16 "partitions" (more on this later) from `DataSourceExec`, which applies a first pass at filtering during the scan.
 2. Next, the output is filtered using `FilterExec` to ensure only rows where `starts_with(URL, 'http://domcheloveplanet.ru/')` evaluates to true are passed on
 3. The `CoalesceBatchesExec` then ensures that the data is grouped into larger batches for processing
 4. The `ProjectionExec` then projects the data to rename the `WatchID` and `ClientIP` columns to `wid` and `ip` respectively.