Fix typo in LRU cache solution (donnemartin#182)

Author: veloxy

solutions/object_oriented_design/lru_cache/lru_cache.ipynb

@@ -21,7 +21,7 @@
     "## Constraints and assumptions\n",
     "\n",
     "* What are we caching?\n",
-    "    * We are cahing the results of web queries\n",
+    "    * We are caching the results of web queries\n",
     "* Can we assume inputs are valid or do we have to validate them?\n",
     "    * Assume they're valid\n",
     "* Can we assume this fits memory?\n",