Update SSD_index.md

Added cosine as one of the supported distance metrics.

workflows/SSD_index.md

@@ -7,7 +7,7 @@ To generate an SSD-friendly index, use the `apps/build_disk_index` program.
 The arguments are as follows:
 
 1. **--data_type**: The type of dataset you wish to build an index on. float(32 bit), signed int8 and unsigned uint8 are supported. 
-2. **--dist_fn**: There are two distance functions supported: minimum Euclidean distance (l2) and maximum inner product (mips).
+2. **--dist_fn**: Three distance functions are supported: cosine distance, minimum Euclidean distance (l2) and maximum inner product (mips).
 3. **--data_file**: The input data over which to build an index, in .bin format. The first 4 bytes represent number of points as an integer. The next 4 bytes represent the dimension of data as an integer. The following `n*d*sizeof(T)` bytes contain the contents of the data one data point in time. `sizeof(T)` is 1 for byte indices, and 4 for float indices. This will be read by the program as int8_t for signed indices, uint8_t for unsigned indices or float for float indices.
 4. **--index_path_prefix**: the index will span a few files, all beginning with the specified prefix path. For example, if you provide `~/index_test` as the prefix path, build  generates files such as `~/index_test_pq_pivots.bin, ~/index_test_pq_compressed.bin, ~/index_test_disk.index, ...`. There may be between 8 and 10 files generated with this prefix depending on how the index is constructed.
 5. **-R (--max_degree)**  (default is 64): the degree of the graph index, typically between 60 and 150. Larger R will result in larger indices and longer indexing times, but better search quality.