fix "an asset definitions" typos (#21609)

## Summary & Motivation

My b

## How I Tested These Changes

docs/content/concepts/assets/software-defined-assets.mdx

@@ -52,7 +52,7 @@ An asset definition includes the following:
 
 ### Basic asset definitions
 
-The easiest way to create an asset definitions is with the <PyObject object="asset" decorator /> decorator.
+The easiest way to create an asset definition is with the <PyObject object="asset" decorator /> decorator.
 
 ```python file=/concepts/assets/basic_asset_definition.py
 import json

docs/content/guides/dagster/enriching-with-software-defined-assets.mdx

@@ -386,7 +386,7 @@ defs = Definitions(
 
 Here's an equivalent job using asset definitions.
 
-**Note:** Because some ops don't correspond to assets, this job uses <PyObject object="op" decorator /> and <PyObject object="graph" decorator /> APIs and <PyObject object="AssetsDefinition" method="from_graph" /> to wrap a graph in an asset definitions:
+**Note:** Because some ops don't correspond to assets, this job uses <PyObject object="op" decorator /> and <PyObject object="graph" decorator /> APIs and <PyObject object="AssetsDefinition" method="from_graph" /> to wrap a graph in an asset definition:
 
 ```python file=/guides/dagster/enriching_with_software_defined_assets/sda_graph.py
 from pandas import DataFrame