fix grammatical errors in 01_intro.ipynb (fastai#407)

* fix grammatical error in 01_intro.ipynb

change "we define a function, 'is_cat', labels cats" to "we define a function, 'is_cat', which labels cats"

* fix grammar

no comma needed after have since the clause after have is not a complete sentence

* fix grammar

change "check will" to "checks will"

01_intro.ipynb

@@ -1503,7 +1503,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "In the third line we define a function, `is_cat`, labels cats based on a filename rule provided by the dataset creators:\n",
+    "In the third line we define a function, `is_cat`, which labels cats based on a filename rule provided by the dataset creators:\n",
     "```python\n",
     "def is_cat(x): return x[0].isupper()\n",
     "```"
@@ -1513,7 +1513,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "We use that function in the fourth line, which tells fastai what kind of dataset we have, and how it is structured:\n",
+    "We use that function in the fourth line, which tells fastai what kind of dataset we have and how it is structured:\n",
     "\n",
     "```python\n",
     "dls = ImageDataLoaders.from_name_func(\n",