Fix typo

Author: ewinge

solutions/solutions_03.ipynb

@@ -69,7 +69,7 @@
     "\n",
     "We will continue using the dictionary `library` from the previous exercise.\n",
     "\n",
-    "Now, use a for-loop to print out all the bibliographic data in the `library` dictionary  by iterating over the keys of the dictionaries.\n",
+    "Now, use `for`-loops to print out all the bibliographic data in the `library` dictionary  by iterating over the keys of the dictionaries.\n",
     "\n",
     "You will need to use two nested `for`-loops.\n",
     "\n",